Anchors & Cable

Tuesday, 07 September 2010 04:46 | Written by Administrator

“Holding on” in an unfamiliar anchorage in bad weather can be the most anxious time of any cruise. It is foolish and false economy to proceed on a cruise with anchors & cable inadequate for the very varied bottom conditions and weather that will be encountered during any voyage. It is a common sight in many vessels that the ground tackle supplied is often far to light for the conditions an ocean going passage maker will be expected to anchor in. The types of bottom most often encountered in anchorages are one of the following:

1) Mud 2) Sand 3) Rock 4) Rock overlaid with sand or mud

At one time or another each of these bottom types will be encountered so the type of anchor should be suitable, as far as possible, for each of these conditions. Broadly, anchors are of two types:

1) Stock type 2) Stockless type

The old style ‘Pick” or Kedge anchor, the Danforth, Moorfast and Stato anchors are all stock anchors. The stock in this type of anchor is designed to keep the anchor upright on the bottom. The big disadvantage with stock anchors is they are difficult to stow neatly on deck. This problem is overcome with the pick anchor by making the stock portable. Most other anchors are of the stockless type; their stability on the bottom is dependant on the size and weight of their “crown” or “head”. The following is a diagram showing the most common types:

Of the types shown the Hall, Spek, AC 14, Pool, Danforth, Bruce and the CQR anchors are the most suitable for Trawler Yachts.

Holding Power

The efficiency of an anchor is expressed as its holding power in lbs versus its weight in lbs. The holding power is proportional to its projected fluke area, shear strength of bottom material and depth of penetration of the anchor into the bottom. The following is a table showing the approx. holding power, for comparison purposes, of each anchor type in mud and sand:

Holding Power

Anchor

Mud

Sand

Hall

3:1

7:1

Spek

3:1

7:1

AC14

5:1

12:1

Pool

6:1

14:1

Danforth

9:1

20:1

CQR

9:1

20:1

Bruce

9:1

20:1

The Hall and Spek type are classed as low Holding Power (LHP) anchors and are most commonly seen on merchant ships.

The AC14 and Pool types are classed as High Holding Power (HHP) anchors.

The Danforth, CQR and Bruce anchors are classed as Very High Holding Power (VHP) anchors.

On the face of it the best choice of anchor for a Trawler Yacht would be one of the VHP types and certainly this is the case for bottom types 1 & 2. The trouble is in the other bottom types 3 & 4. No anchor can develop any bottom penetration and an anchors only asset in this situation is its dead weight. A better choice would be one of the HHP types which are smaller physically weight for weight than the VHP types. The HHP types are also much easier to stow on deck or in a hawse pipe. Weight of Anchor & Cable The best approach for assessing the weight of anchors and chain sizes is to use one of the Classification Societies Rules such as Lloyds. Each Society has formulae to calculate the Equipment Numeral (N.A.) for all types of Ships and Navigation Notations. The N.A. formulae is a very complex calculation which takes into account the vessels displacement, beam & length, frontal & profile areas, number of decks and freeboards. A table is provided from which the required number of anchors, their weight and the size and length of chain cable is derived.

The following is an example of one of these tables. Windlass

21. As a rule a windlass of suitable power for the mass of anchors and the size of the chain is to be securely attached to the deck which is to be efficiently strengthened and pillared.

22. The windlass is to be installed so that the chain run between the hawse pipes and the chain locker is as direct as possible. The moving parts of the windlass that are likely to cause accidents are to be properly guarded.

NA

Stockless Anchors

Chain Cables

Greater than

Less than or equal to

Number

Mess (kg)

Length of each mooring line (m)

Nominal Diameter (mm)

Studless Chain Cable

Stud Link chain cables

01

02

10

12

1

15

35

7

12

14

1

19

40

7

14

16

1

23

45

8

16

18

1

26

50

8

18

20

1

30

55

9

20

23

2(1)

33

60

9

23

26

2(1)

39

65

10

26

30

2(1)

45

70

10

30

35

2(1)

52

75

11

35

40

2(1)

62

80

12

40

45

2(1)

72

85

13

45

50

2

82

90

14

12.5

50

60

2

91

95

14

12.5

60

70

2

113

100

15

14

12.5

70

80

2

135

105

16

14

12.5

80

90

2

155

110

16

14

90

100

2

180

110

16

14

100

110

2

205

125

16

14

110

120

2

230

125

19

17.5

120

130

2

255

140

19

17.5

130

140

2

128

140

19

17.5

140

150

2

310

140

19

17.5

150

170

2

340

140

20.5

17.5

170

190

2

405

150

20.5

17.5

190

210

2

455

150

22

19

210

230

2

510

150

22

19

230

250

2

550

150

24

20.5

250

300

2

600

150

26

22

In the example in bold it can be seen that a vessel with N.A. between 70-80 requires 2 x 135kg (300lbs) anchors and 105 metres (345ft) of 14mm (9/16ths) chain.

The anchor weights in the table are for L.H.P. anchors. A reduction factor of 0.75 is allowed if a H.H.P. anchor is chosen. Q1 chain is mild steel and Q2 chain is high tensile steel. H.H.P. anchors must be coupled with Q2 chain.

Therefore the vessel in question could be equipped with the following: 2 x 100kg (220lb) Pool (N) Anchors 105metres (345ft) 12.5mm (1/2 inch) Q2 Cable This would provide holding power in mud of 600kg (1323lbs) and 1400kg (3100lbs) in sand.

The Watson 48™ for example has an N.A. of 50 and is specified with the following: 2 – 60Kg (132lb) Pool “N” High Holding Power Anchors. 180 metres (590ft) 13mm (1/2 inch) Short Link Chain.

Anchor Stowage

Anchors much over 60lbs are impossible to man handle and must self stow. This can be done with difficulty, by fitting a tilting bow roller system. By far the best is to have the anchors self stow in hawse pipes in the hull. This is not possible with most V.H.P types because they cannot be made to stow neatly against the hull. In steel vessels the hawse pipes need to be stainless steel with a stainless steel striker plate against the hull for the anchor to stow against. Yachts should always have both anchors connected for immediate deployment. Having the second (spare) anchor stowed on deck is not good practice.

Winches

For vessels up to about 60ft yacht windlasses such as those made by Maxwell and Muir are excellent except that they are not strong enough to ride on and must be fitted with chain stoppers. 24VDC electric systems are fine, however if the ship has a hydraulic system then this may be the preference. Larger vessels almost inevitably need custom built windlasses.

Chain Lockers

A Yacht should have watertight, self stowing chain lockers located immediately below the windlass spurling pipes. For safety their drains must be connected to the ships central bilge pumping system. Stud link chain is always preferable for stowing as it does not become knotted in the chain locker. In conclusion a ships anchoring gear, including the windlass, is of prime importance; it is always better to “kick for touch” when assessing anchor and cable requirements for an ocean passage maker.